HI Observations of Galaxies in the Southern Filament of the Virgo

Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 20 January 2018 (MN LATEX style file v2.2) HI observations of galaxies in the southern filament of the Virgo Cluster with the SKA Pathfinder KAT-7 and the WSRT A. Sorgho1⋆, K. Hess1,2,3, C. Carignan1,4, T. A. Oosterloo2,3 1 Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa 2 Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands 3 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands 4 Observatoire d’Astrophysique de l’Universitéde Ouagadougou (ODAUO), Ouagadougou, Burkina Faso ABSTRACT ◦ ◦ We map the HI distribution of galaxies in a ∼ 1.5 × 2.5 region located at the virial radius south of the Virgo cluster using the KAT−7 and the WSRT interferometers. Because of the ∼ different beam sizes of the two telescopes, a similar column density sensitivity of NHI 1 × 1018 atoms cm−2 was reached with the two observations over 16.5 km s−1. We pioneer a new approach to combine the observations and take advantage of their sensitivity to both the large and small scale structures. Out to an unprecedented extent, we detect an HI tail of ∼ 60 kpc being stripped off NGC 4424, a peculiar spiral galaxy. The properties of the galaxy, together with the shape of the tail, suggest that NGC 4424 is a post-merger galaxy undergoing a ram pressure stripping as it falls towards the centre of the Virgo Cluster. We detect a total of 14 galaxies and 3 HI clouds lacking optical counterparts. One of the clouds is a new detection 7 −1 with an HI mass of 7 × 10 M⊙ and a strong HI profile with W50 = 73 km s . We find that 10 out of the 14 galaxies present HI deficiencies not higher than those of the cluster’s late spirals, suggesting that the environmental effects are not more pronounced in the region than elsewhere in the cluster. Key words: techniques: interferometric – galaxies: clusters – individual: Virgo – galaxies: ISM – individual: NGC 4424 1 INTRODUCTION include NGC 4388 (Oosterloo & van Gorkom 2005), NGC 4522 (Kenney, van Gorkom & Vollmer 2004; Vollmer et al. 2004), NGC When a galaxy resides in a dense region such as a cluster or a group, 4402 (Crowl et al. 2005) and NGC 4438 (Chemin et al. 2005). Al- it undergoes different mechanisms that determine its morphology though this effect is strongest in the central regions of clusters, cos- arXiv:1609.03896v1 [astro-ph.GA] 13 Sep 2016 (e.g. Dressler 1980) and gas properties (Cayatte et al. 1990). Galax- mological simulations reveal that it remains an important mecha- ies evolving in dense environments tend to be HI deficient with nism of gas stripping of galaxies out to the virial radius of clusters respect to their morphological counterparts evolving in less dense (Tonnesen, Bryan & van Gorkom 2007). regions (Haynes, Giovanelli & Chincarini 1984). More specifically in clusters of galaxies, the small scale structure interstellar medium While the optical characteristics of galaxies in dense environ- (ISM) of spiral galaxies interacts with the large scale structure in- ments seem well understood, we are still far from a complete pic- tracluster medium (ICM) through hydrodynamical processes that ture of galaxy evolution. Observations of the neutral atomic gas in then drive the evolution of those galaxies in clusters. Perhaps the dense environments are key to our understanding of the growth and most important mechanism at work is the ram pressure (RP) strip- depletion of gas disks, as well as star formation quenching in galax- ping (e.g. Gunn & Gott 1972; Vollmer et al. 2001). It occurs when ies. Further, they provide tests for numerical simulations and con- a galaxy moves through the ICM of a galaxy cluster and the ex- straints for modelling galaxy evolution (e.g. Davéet al. 2013). In ternal force felt by the ISM is greater than the gravitational restor- particular the neutral gas provides an opportunity to map the outer ing force from the galaxy disk. It is known to be partly respon- regions of galaxies and their surrounding environment, allowing sible for the HI deficiency (Bothun, Sullivan & Schommer 1982; us to directly probe their interactions with the IGM and with their Giovanelli & Haynes 1985; Vollmer et al. 2001) and star forma- neighbours. tion quenching (e.g. Balogh, Navarro & Morris 2000). Typical ex- To date, about 1300 galaxies have been identified, based amples of galaxies in the Virgo cluster undergoing RP stripping on their morphology and radial velocities, as true members of the Virgo Cluster (e.g. Sandage, Binggeli & Tammann 1985; Binggeli, Tammann & Sandage 1987). Located at 16.8 Mpc (dis- ⋆ [email protected] tance of M87, Neill et al. 2014), the recession velocity of the clus- c 0000 RAS 2 Sorgho et al. ter is 1050 km s−1, and its velocity dispersion is 700 km s−1∼(Binggeli, Popescu & Tammann 1993). Detailed X-ray∼ observations of the hot cluster gas (Böhringer et al. 1994) revealed several substructures, suggesting that the cluster is not virialised, as seen in other clusters (see e.g. for the Coma & Antlia clusters, Briel, Henry & Boehringer 1992; Hess et al. 2015). The Virgo Cluster, instead, is still growing. Subclusters around the ellipticals M86 and M49 (whose systemic velocities are respectively -181 and 950 km s−1, Kim et al. 2014) are merging with the main cluster around the central elliptical M87. Several HI imag- ing surveys of the cluster revealed that the HI disks of its central late-type galaxies are truncated to well within the optical disks, suggesting that ICM - ISM interactions play an important role in driving the evolution of galaxies in the inner region of the cluster (Warmels 1988; Cayatte et al. 1990; Chung et al. 2009; Boselli et al. 2014). Wide-area HI surveys like HIPASS (HI Parkes All-Sky Survey, Barnes et al. 2001), ALFALFA (Arecibo Legacy Fast ALFA, Giovanelli et al. 2005) and AGES (Arecibo Galaxy Environment Survey, Auld et al. 2006) done with single- dish telescopes have helped detect many low HI mass galaxies such as early-type and late-type dwarf galaxies, but also starless “dark” galaxies and several HI clouds (e.g., Alighieri et al. 2007; Kent et al. 2007). In particular, ALFALFA observations allowed the detection of complex structures associated with Virgo galaxies (e.g. Koopmann & Giovanelli 2008), showing probable signs of interactions in the cluster. We use the KAT-7 and the WSRT to study galaxies located in an X-ray filament to the South-West of the cluster connecting the main cluster with the substructure associated with M49. We Figure 1. X-ray map of the Virgo Cluster (0.5 − 2.0 keV, ROSAT; observe a 1.5◦ 2.5◦ region in the M49 subcluster. It is located Böhringer et al. 1994). The circle shows the virial radius of the cluster ∼ × ◦ . ◦ south of the Virgo Cluster, 3 away from the elliptical M87, (∼ 3 7 or 1.08 Mpc, Arnaud, Pointecouteau & Pratt 2005; Urban et al. 2011) around M87, and the box is the region observed in the present work. and contains the elliptical M49∼ (see Fig. 1). The field also contains The labelled plus signs show the major ellipticals of the cluster, and the the one-sided HI tail galaxy, NGC 4424, previously observed using cross indicates the position of the HI-tailed galaxy NGC 4424. the VLA (Chung et al. 2007, 2009). Taking advantage of the short baselines of the KAT-7 telescope sensitive to extended structures and the high spatial resolution of the WSRT telescope, we map late- to calibrate the phase stability. The choice of these calibrators lies type galaxies in the region down to low column densities. Given the on their close angular distance from the target field and their good large difference in the resolutions of the two arrays, we develop a quality status as given by the ATCA Calibrator Database1 and the new technique to combine the data from the two telescopes. VLA Calibrator Manual2. The calibrator must be as close as possi- We begin with a description of the observations with the two ble to the target, and have a good quality rating in the 21cm L-band. telescopes, as well as the technique adopted for the data combina- We used three pointings to cover the desired field, uniformly tion in section 2. The results are presented in section 3 where the separated (by 72% of the primary beam) and following the line HI map of the region as well as the HI parameters of the detections connecting the∼ objects NGC4424 and M49 (see Fig. 2). The upper are given. In section 4 we discuss the results in light of the cluster pointing was observed for 30 hours split in 6 sessions, while the environment effects, and give a summary in section 5. The contours two others were given 24∼ hours each during a total of 8 sessions. and HI profiles of all the detections are presented in the Appendix. Initially, the observations were focused on NGC 4424 and its HI tail; however, to have a complete view of the tail and to probe the whole region between the galaxy and the elliptical M49, the upper pointing was expanded to a mosaic. 2 OBSERVATIONS AND DATA REDUCTION During these observations, KAT-7 was still in the commis- 2.1 KAT-7 observations and reduction sioning phase and the RFI monitoring was still ongoing, there- fore special care needed to be given to the inspection of the data The Karoo Array Telescope (KAT-7) observations were conducted beforehand.

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